1. Field of the Invention
The present invention relates generally to marine seismology, in which a moving ship generates seismic waves and detects reflections. Still more particularly, the invention relates to the correction of the detected seismic waves for the motion of the ship.
2. Background of the Invention
The field of seismology focuses on the use of artificially generated elastic waves to locate mineral deposits such as hydrocarbons, ores, water, and geothermal reservoirs. Seismology also is used for archaelogical purposes and to obtain geological information for engineering. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data can provide information about the structure and distribution of rock types and their contents.
Most oil companies rely on seismic interpretation for selecting the sites in which to invest in drilling exploratory oil wells. Despite the fact that seismic data is used to map geological structures rather than finding petroleum directly, the gathering of seismic data has become a vital part of selecting the site of an exploratory and development well. Experience has shown that the use of seismic data greatly improves the likelihood of a successful venture.
Seismic data acquisition is routinely performed both on land and at sea. At sea, seismic ships deploy a streamer or cable behind the ship as the ship moves forward. The streamer includes multiple receivers in a configuration generally as shown in FIG. 1. Streamer 110 trails behind ship 100 which moves in the direction of the arrow 101. As shown in FIG. 1, source 112 is also towed behind ship 100. Source 112 and receivers 114 typically deploy below the surface of the ocean 70. Streamer 110 also includes electrical or fiber-optic cabling for interconnecting receivers 114, and the seismic equipment on ship 100. Streamers are usually constructed in sections 25 to 100 meters in length and include groups of up to 35 or more uniformly spaced receivers. The streamers may be several miles long and often a seismic ship trails multiple streamers to increase the amount of seismic data collected. Data is digitized near the receivers 114 and is transmitted to the ship 100 through the cabling at rates of 7 (or more) million bits of data per second. Processing equipment aboard the ship controls the operation of the trailing source and receivers and processes the acquired data.
Seismic techniques estimate the distance between the ocean surface 70 and subsurface structures, such as structure 60 which lies below the ocean floor 63. By estimating the distance to a subsurface structure, the geometry or topography of the structure can be determined. Certain topographical features are indicative of oil and/or gas reservoirs.
To determine the distance to subsurface structure 60, source 112 emits seismic waves 115 which reflect off subsurface structure 60. The reflected waves are sensed by receivers 114. By determining the length of time that the seismic waves 115 took to travel from source 112 to subsurface structure 60 to receivers 114, an estimate of the distance to subsurface structure 60 can be obtained.
The receivers used in marine seismology are commonly referred to as hydrophones, or marine pressure phones, and are usually constructed using a piezoelectric transducer. Synthetic piezoelectric materials, such as barium zirconate, barium titanate, or lead mataniobate, are generally used. A sheet of piezoelectric material develops a voltage difference between opposite faces when subjected to mechanical bending. Thin electroplating on these surfaces allows an electrical connection to be made to the device so that this voltage can be measured. The voltage is proportional to the amount of mechanical bending or pressure change experienced by the receiver as resulting from seismic energy propagating through the water. Various types of hydrophones are available such as disk hydrophones and cylindrical hyrdophones.
Two types of seismic sources are used to generate seismic waves for the seismic measurements. The first source type comprises an impulsive source which generates a high energy, short time duration impulse. The time between emitting the impulse from the source and detecting the reflected impulse by a receiver is used to determine the distance to the subsurface structure under investigation. The impulsive source and the associated data acquisition and processing system are relatively simple. However, the magnitude of energy required by seismic techniques using impulsive sources may, in some situations, be harmful to marine life in the immediate vicinity of source 112.
The environmental concerns associated with impulsive sources has lead to the use of another type of seismic source which generates a lower magnitude, vibratory energy. The measurement technique which uses such a source is referred to as the marine vibratory seismic ("MVS") technique. Rather than imparting a high magnitude pressure pulse into the ocean in a very short time period, vibratory sources emit lower amplitude pressure waves over a time period typically between 5 and 7 seconds, but longer time periods are also possible. Further, the frequency of the vibrating source varies from about 5 to 150 Hz, although the specific low and high frequencies differ from system to system. The frequency of the source may vary linearly with respect to time or non-linearly. The frequency variations are commonly called a "frequency sweep". The frequency sweep is thus between 5 and 150 Hz and 5 to 7 seconds in duration. The magnitude of the seismic wave oscillations may vary or remain at a constant amplitude. The amplitude of the oscillations, however, are much lower than the magnitude of impulsive sources and thus, there are fewer environmental concerns with the MVS seismic technique.
Seismic ships must move forward while seismic measurements are being recorded for many reasons. Referring still to FIG. 1, the hydrophones 114, connecting wires and stress members provided on the streamers are placed inside a neoprene tube (not shown in FIG. 1) 2.5-5 inches in diameter. The tube is then filled with sufficient lighter-than-water liquid to make the streamer neutrally buoyant. A lead-in section 111 of the streamer 110 approximately 300 meters long and a number stretch of sections approximately 50 meters long trail between the ship's stern and the streamer section 116 in which the receivers 114 are included. A diverter 113 pulls the streamer section 116 out to an appropriate operating width. Depth controllers (not shown) are fastened to the streamer at various places along its length. These devices sense the hydrostatic pressure and tilt bird wings so that the flow of water over them raises or lowers the streamer to the desired depth. The depth that the controllers seek to maintain can be controlled by a signal sent through the streamer cabling and thus the depth can be changed as desired. For the streamer's depth control system to function effectively, the ship 100 must travel forward at a speed through the water of approximately four knots.
Second, streamer 110 usually is a flexible cable and thus the ship must move forward to maintain a desired fixed separation between the sources and streamers, and between the streamers themselves. The spacing between sources and streamers is important in the marine seismology and must not vary while seismic measurement are made.
Third, seismic ships often deploy multiple streamers using diverters that allow a fixed separation to be maintained between streamers. These diverters force the streamers laterally as the boat moves forward. Without the barvanes, the streamers may become entangled. The relative velocity of the water around the diverters and the angle of attack determine the amount of separation between streamers.
Fourth, seismic ships must cover as much ocean surface as possible each day because of the cost of operating the ship. For these reasons and others, seismic ships must move forward while taking measurements and the forward speed must be reasonably constant. Typical ship speed is approximately 2-3 meters per second. Because the streamer is deployed behind the ship, the source and receivers also move at approximately 2.5 meters per second.
Marine seismic measurements can also be made using a technique called "on-bottom cable" (OBC) in which a ship lays one or more cables containing hydrophones and geophones on the ocean floor. This ship remains stationary and records data while collecting seismic data. The second ship containing sources moves parallel, or at some other angle, to the cables. In the OBC technique, the receivers do not move, but the sources are moving and thus, the acquired data is distorted. Further, in special circumstances, some of the receivers can be on land. Although OBC is generally more expensive than towed marine seismic measurements, OBC is necessary if land obstructions, such as an island, are located where the cables are to be layed.
Although ship motion is necessary as described above, the motion distorts or "smears" the acquired seismic data. Broadly, smearing results from the fact that the ship, and thus the sources and receivers, move while data collection takes place. It is generally recognized that the smearing effect of ship motion on seismic data results from two analytically separate phenomena--source motion and receiver motion. Although the receivers and source are pulled behind the ship and thus move at the same speed as the ship, the effect of source motion on the data is usually analyzed independently from the effect of receiver motion. Source motion is less of a concern than receiver motion in impulsive source-based seismic systems because the source moves a negligible amount during the brief impulse emitted by the source. Data smearing in a MVS system includes significant contributions from both receiver and source motion. Thus, the MVS-acquired data should be corrected for both receiver and source motion.
The high costs associated with operating a seismic ship require that the methods and procedures used be efficient. It is thus desirable to maximize data collection in as short a time as possible. Because of the length of the frequency sweep (typically 5 seconds or more), MVS sources are typically activated every 10 to 20 seconds. Because of the ship's speed (2-3 meters per second), a MVS source must be activated no sooner than every 12.5 to 75 meters. Although more data in one location could be acquired if the ship were to travel at a slower speed, streamer control would be lost and less ocean surface would be covered each day, thereby increasing the cost required to make seismic measurements of a desired section of the subsurface.
At least one attempt has been made to correct for receiver and source motion for MVS recorded data. In an article entitled "The Effects of Source and Receiver Motion on Seismic Data," by Hampson and Jakubowicz, Geophysical Prospecting, 1995, p. 221-244, a method for correcting for receiver and source motion is disclosed. Although the method of Hampson and Jakubowicz has theoretical merit, the method is impractical for use with conventional marine seismic systems as it requires the MVS source to be activated with a temporal and spatial spacing that is impractical. It is well known that for a wave traveling with a velocity V through a medium such as water and with a frequency of F (i.e., the number of complete cycles of the waveform per second), the velocity V is related to the frequency F by the length of the wave, referred to as the wavelength (.lambda.). The relationship is: EQU V=F.multidot..lambda.. (1)
Thus, the wavelength .lambda. is V/F. In water seismic waves propagate with a known velocity of approximately 1500 meters per second (approximately 3325 miles per hour). If the highest frequency in a sweep is assumed to be 60 cycles per second (or 60 "Hz"), the wavelength of such a seismic wave is 25 meters (1500/60). To avoid a certain type of data distortion known as "aliasing", the source must be activated at a spacing of at least one half of the wavelength. Thus, for Hampson and Jakubowicz's method to work the vibratory sources must be activated at least every 12.5 meters, and preferably sooner. To activate a source at such narrow spacings, the ship must travel much slower than its preferred 2-3 meters per second.
It would be advantageous to provide a practical seismic system for use in marine applications that can correct the data for the motion of the ship without the deficiency inherent in the Hampson and Jakubowicz method. Such a system preferably would correct for both receiver and source motion and do so in a cost effective manner. Despite the apparent advantages, to date all attempts of developing such a system have failed.